Anti-skid systems are used on land vehicles such as trucks to prevent the vehicle from skidding when the brakes are rapidly applied in an emergency situation or the roads are slippery or wet. Each wheel of the vehicle is equipped with a sensor which senses the speed of the wheel. The signal representative of the speed of the wheel is applied by a tone wheel which has a specified number of teeth, or webs. The sensor includes a coil, and the tone wheel is ferromagnetic so that the teeth of the wheel induce signals into the coil of the sensor. The output voltage of the sensor is therefore a sinusoidal wave, the frequency of which is directly proportional to the speed of the wheel. When the brakes of a vehicle are applied rapidly in emergency situations, the speed of the wheels should decrease substantially, rapidly, and very uniformly; however, if the vehicle is about to go into a spin, the one wheel locks, that is, the velocity goes to zero. The sensor associated with the locked wheel detects the absence of rotation and supplies a signal to the anti-skid system which then reacts to release the brake of the locked wheel and prevent the vehicle from going into a skid.
The invention described in application Ser. No. 632,095, filed Nov. 14, 1975, fully referenced hereinbelow, is directed to a system for varifying that the sensor and tone wheel of the anti-skid system are functioning and are connected properly. The testing system described in the cross-referenced patent application therefore measures the resistance of the sensor to make certain that the coil which responds to the webs of the tone wheel is proper and is supplying voltages of the proper level to the anti-skid system. The testing system also verifies that the tone wheel has no missing teeth, is properly aligned, and is the proper distance from the speed sensor. The invention described herein therefore simulates these signals to verify that the testing apparatus is properly functioning and yielding reject signals when signals indicative of a bad sensor or tone wheel should be yielded by the testing device. The invention is also useful in calibrating the testing system described in the cross-referenced application.